1. Field of the Invention
The present invention relates to a method and apparatus for reducing sheet material curl induced in a fusing operation.
2. Description of Related Art
In an electrophotographic (EP) image forming apparatus, such as a printer or copier, a latent image is formed on a light sensitive drum and developed with an appropriate color toner. For some color printers, the toner image may then be transferred onto an intermediate transfer member (ITM) belt. For both mono and color printers, the toner image is transferred to a substrate, such as a sheet of paper. The substrate with the toner image is subsequently passed through a fuser where heat is applied to melt the toner and fuse it to the substrate. The fuser includes a hot roller cooperating with a backup roller to form a nip through which the toned substrate passes. The hot roller is provided with an internal heater, such as a tungsten-filament lamp, for generating heat, which is transferred to an outer surface of the hot roller. A temperature sensor is provided for sensing the temperature of the hot roller outer surface. The sensor generates a temperature signal to a print engine or processor for controlling the temperature of the hot roller outer surface to a predetermined target temperature.
Additionally, in order to facilitate fuser warm-up and temperature control, it is known to provide the backup roller with an internal heater for generating heat, which is transferred to an outer surface of the backup roller. An additional temperature sensor is provided for sensing the temperature of the backup roller outer surface.
In order to minimize the cost of the fuser assembly, it would be desirable to provide a heater element and temperature sensor for only the hot roller. However, when only the hot roller includes a heater element, undesirable amounts of negative longitudinal curl around a longitudinal axis of the substrate, such as a sheet of paper, may occur due to a temperature differential between the hot roller and the backup roller, i.e., the substrate curls about its longitudinal axis away from the fused toner image such that the toner image is on the outer surface of the curled substrate. For some printers, such as some mono printers, the hot roller may press into the backup roller causing a large amount of positive width curl which helps minimize fuser induced negative longitudinal curl. However, other printers, such as some color printers, generally need an aggressive reverse nip in which the backup roller presses into the hot roller to provide desirable release characteristics, resulting in negative width curl. Therefore, these other printers can not use the same technique to minimize the induced negative longitudinal curl.
Accordingly, there is a need for other techniques to reduce sheet material curl induced by a fuser assembly.